Enhancing the solubility of 1,4-diaminoanthraquinones in electrolytes for organic redox flow batteries through molecular modification

C-H···π interactions disrupt electrostatic interactions between non-aqueous electrolytes to increase solubility

Grid-scale energy storage applications, such as redox flow batteries, rely on the solubility of redox-active organic molecules. Although redox-active pyridiniums exhibit exceptional persistence in multiple redox states at low potentials (desirable properties for energy storage applications), their solubility in non-aqueous media remains low, and few practical molecular design strategies exist to improve solubility. Here we convey the extent to which discrete, attractive interactions between C-H groups and π electrons of an aromatic ring (C-H···π interactions) can describe the solubility of N-substituted pyridinium salts in a non-aqueous solvent. We find a direct correlation between the number of C-H···π interactions for each pyridinium salt and its solubility in acetonitrile. The correlation presented in this work highlights a consequence of disrupting strong electrostatic interactions with weak dispersion interactions, showing how minimal structural change can dramatically impact pyridinium solubility.

Explainable Solvation Free Energy Prediction Combining Graph Neural Networks with Chemical Intuition

The prediction of a molecule's solvation Gibbs free (ΔG_solv) energy in a given solvent is an important task which has traditionally been carried out via quantum chemical continuum methods or force field-based molecular simulations. Machine learning (ML) and graph neural networks in particular have emerged as powerful techniques for elucidating structure-property relationships. This work presents a graph neural network (GNN) for the prediction of ΔG_solv which, in addition to encoding typical atom and bond-level features, incorporates chemically intuitive, solvation-relevant parameters into the featurization process: semiempirical partial atomic charges and solvent dielectric constant. Solute-solvent interactions are included via an interaction map layer which can be visualized to examine solubility-enhancing or -decreasing interactions learnt by the model. On a test set of small organic molecules, our GNN predicts ΔG_solv in water and cyclohexane with an accuracy comparable to polarizable and ab initio generated force field methods [mean absolute error (MAE) = 0.4 and 0.2 kcal mol^-1, respectively], without the need for any molecular simulation. For the FreeSolv data set of hydration free energies, the test MAE is 0.7 kcal mol^-1. Interpretability and applicability of the model is highlighted through several examples including rationalizing the increased solubility of modified diaminoanthraquinones in organic solvents. The clear explanations afforded by our GNN allow for easy understanding of the model's predictions, giving the experimental chemist confidence in employing ML models toward more optimized synthetic routes.

Computational Studies Followed by Effect of Solvent Polarity and Salts on HOMO-LUMO Gap of 7-Hydroxy Coumarine Notabally Reflected by Absorption and Emission Spectra

7-Hydroxycoumarin's FT-IR solid phase spectra were observed at 4000-400 cm^-1. The spectra were analyzed in aspects of significant approaches. DFT was used to optimize the structure of the compound and its structural properties. The molecular properties were also determined by the HF/3-21G level. The bond lengths and bond angles were obtained by the computational study of the optimized geometry. The vibrational frequencies were determined in all these approaches, which were then matched to experimental frequencies, yielding an excellent agreement between measured and estimated frequency ranges. The UV-visible spectrum of 7HC was obtained and the electronic characteristics HOMO and LUMO energies were monitored by the time-dependent TD-DFT method. The spectral behavior of 7-Hydroxycoumarin was studied using fluorescence spectroscopy in a wide range of polar and non-polar solvents. Solvatochromic effect was observed in both the fluorescence and absorption spectra. The structural properties, energies, IR intensities, absorption wavelengths, and harmonic vibrational frequencies were compared with the obtainable experimental information of the molecule.

Mixture of Anthraquinone Sulfo-Derivatives as an Inexpensive Organic Flow Battery Negolyte: Optimization of Battery Cell

Anthraquinone-2,7-disulfonic acid (2,7-AQDS) is a promising organic compound, which is considered as a negolyte for redox flow batteries as well as for other applications. In this work we carried out a well-known reaction of anthraquinone sulfonation to synthesize 2,7-AQDS in mixture with other sulfo-derivatives, namely 2,6-AQDS and 2-AQS. Redox behavior of this mixture was evaluated with cyclic voltammetry and was almost identical to 2,7-AQDS. Mixture was then assessed as a potential negolyte of anthraquinone-bromine redox flow battery. After adjusting membrane-electrode assembly composition (membrane material and flow field)), the cell demonstrated peak power density of 335 mW cm^-2 (at SOC 90%) and capacity utilization, capacity retention and energy efficiency of 87.9, 99.6 and 64.2%, respectively. These values are almost identical or even higher than similar values for flow battery with 2,7-AQDS as a negolyte, while the price of mixture is significantly lower. Therefore, this work unveils the promising possibility of using a mixture of crude sulfonated anthraquinone derivatives mixture as an inexpensive negolyte of RFB.

Novel Series of Diaminoanthraquinone-Based π-Extendable Building Blocks with Tunable Optoelectronic Properties

We have achieved the first series of DAAQ-based building blocks, viz., n -TIPS-DAAQs (n = 1-4), and unraveled a rational design of their π-extension. Sequentially increasing numbers (n) of the exocyclic π-linkers showed (a) a systematic bathochromic shift in both absorption and emission spectra, (b) selective stabilization of the lowest-unoccupied molecular orbital (LUMO), and (c) unselective changes in the S₀/S₁ states. To our surprise, the LUMO level of 4-TIPS-DAAQ (-3.72 eV) was found to be comparable to that of PC₆₀BM.